When installing a stinger assembly into a bore in a tubing hanger (TH), there are some compromises to consider which is discussed below. Increasing production bore size reduces space through the tubing hanger for the annulus bore; this reduces available annulus flow area. In order to achieve an acceptable flow area in a reduced bore annulus stinger, it is typical to split the annulus bore into 2 separate smaller stingers. This is unfavourable from a reliability perspective amongst others.
The ideal solution is to reduce the cross-section of the stinger wall in order to obtain the desired flow area within the available space, but conventionally used metal lip seals do not allow a sufficiently thin cross-section. Stinger integral metal bump seals, which allow a thin cross-section, do not flex sufficiently to make reliable seals and replacement of such a seal on a completed XT assembly at typical small diameters, is a costly task as the whole stinger must be removed.
According to the present invention an oilfield tubular of the introductory said kind is provided, which is distinguished in that the oilfield tubular includes axial sealing means located on an end surface of the oilfield tubular, which sealing means includes a metal c-ring seal, which seal is smaller in cross section than the thickness of the oilfield tubular wall and the opening of the metal c-ring seal is facing the high pressure side of the production bore, which pressure tend to expand the c-profile to firm engagement with contacting surfaces.
Thus the invention relates to the use of an axial sealing means, i.e. an axial sealing between 2 parallel bores. To provide such sealing, the proposed metal c-ring seal is used on the stinger, as opposed to a metal lip-seal. These seals can be made in extremely small cross-sections and thus enable a very thin section stinger.
Preferably, the opening of the metal c-ring seal is facing in the axial direction of the oilfield tubular and away from said oilfield tubular.
Preferably, the axial sealing means on the end surface of the oilfield tubular includes an external ledge or shoulder on the end surface, which shoulder supports the metal c-ring seal.
Preferably the metal c-ring seal makes sealing contact with a bore wall in the tubing hanger which is adapted to receive the oilfield tubular, in addition to make sealing contact with an external surface of the oilfield tubular, which external surface is part of the shoulder and is opposing the bore wall.
Normally, the metal c-ring seal is less than 1-5 mm in cross section.
When using these seals at small cross-sections (e.g. 1-3 mm or even 5 mm) the c-ring gets extremely stiff if the body of the seal is designed to resist normal pressures used for XT systems. A stiff seal is typically not a reliable seal unless seal bore tolerances are reduced to uneconomic levels.
In one embodiment the oilfield tubular shoulder may include a V or U shaped seat, which seat is designed to receive and support the metal c-ring seal.
Therefore a design is proposed where the c-ring is made thinner than it typically needs to be to handle the pressure rating, thus allowing it to be more resilient to tolerance variations. This is combined with the V or U shaped seat provided in the stinger which supports the thin seal and allows it to handle a higher pressure rating.
A similar c-ring seal is used with electrical and hydraulic connectors through the TH. To our knowledge, metal c-rings have never been used in this application and this is attributed to the excessive tight tolerances required, or the inability for typical flat seated c-rings to handle the pressure—a problem that this design solves.
Using the c-ring on the annulus (or production) stinger and adding a U or V shaped seat will improve pressure rating and reduce tolerance sensitivity.
The c-ring operates like a normal lip seal and that is how it is used in most applications. Specifically for this application it works because of (a) small size to achieve the required stinger envelope (b) thin section material to make the seal flexible allowing manufacturing tolerances that can be reliably machined (c) u shaped seat that supports the c-ring against pressure.
With a normal flat seat behind the c-ring pressure tries to make the c-ring go a square shape. The effect of this is that after pressure application the seal is permanently deformed and loses contact preload between the surfaces being sealed unless the material thickness is sufficient to avoid plastic damage.
The U shaped seat supports the seal and means that it cannot be forced by pressure into a square shape. Therefore a thinner than usual seal can be used thus obtaining the benefit (b) given above.
Any typical corrosion resistant alloy can be used here. The seal is very slightly deformed if installed at tolerance extremes and some further small deformation does occur with initial pressure application but after this the c-ring operates in an elastic regime. The seal can be re-used in theory since these deformations are extremely small when compared to the seal interference with its sealing surfaces.
Thus the metal c-ring seal may preferably be made of a corrosion resistant alloy, such as Inconel.
On one embodiment the metal c-ring seal has a wall thickness in the range of 0.01 to 0.25 mm.
On one embodiment the shoulder is an angular notch, preferably a 90 degree notch. This shoulder may be defined by one transversally extending surface and one axially extending surface, both surfaces supporting the metal c-ring seal.
Reference is first made to FIG. 1 showing in general a tubing hanger 2 installed in a wellhead 5 located in a Christmas tree (XMT) on the seabed. These components are well known per se and will not be described in further detail. A stinger 1 is installed in the tubing hanger 2 and provides an independent flow path 3 external to the main flow bore 4 through the wellhead 5. The flow path 3 of the stinger 1 may be used, as an example, to supply fluid or gas from the surface to the well flow to provide gas lift to the produced flow.
Reference is now made to FIG. 2, which is an enlarged view of the lower end area of the stinger 1 in FIG. 1. The stinger 1 includes axial sealing means 6 located at the lower end 1a of the stinger 1. This axial sealing means 6 includes a metal c-ring seal 6a. Further, the axial sealing means 6 at the lower end 1a of the stinger 1 includes an external ledge 1b, or shoulder, which is more clearly shown in FIG. 3. The shoulder 1b supports the metal c-ring seal 6a. 
As shown in FIG. 3 and still more detailed in FIG. 4A to 4D, the shoulder 1b is defined by one transversally extending surface 1b′ and one axially extending surface 1b″. Both surfaces 1b′ and 1b″ support the metal c-ring seal 6a. In addition, the metal c-ring seal 6a makes contact with the bore wall 2a in the tubing hanger 2. The shoulder surfaces 1b′ and 1b″ is typically like an angular notch, normally a 90 degree angle between the surfaces 1b′ and 1b″. 
The opening 6b of the metal c-ring seal 6a is installed in such a way that the opening 6b is exposed to the high pressure side of the production bore 4. When the metal c-ring seal 6a is exposed to such high pressure at the opening 6b, the high pressure tend to expand the c-profile to firm engagement with contacting surfaces, i.e. the supporting shoulder surfaces 1b′ and 1b″ and the bore wall 2a. 
Normally, this means that the opening 6b of the metal c-ring seal 6a is facing in the axial direction of the stinger 1 and away from the stinger 1. This also means that the sealing means 6 is smaller in cross section than the thickness t1 of the stinger wall 1c. 
The shoulder 1b, or ledge, of the stinger 1 will normally include either a V or a U shaped seat s. The seat s is designed to receive and support the metal c-ring seal 6a. Normally, such seat s will be provided in the transversally extending shoulder surface 1b′. However, it would be possible also to provide such seat in the axially extending shoulder surface 1b″, and even the bore wall 2a. 
The theory behind this design and function is further detailed below with reference to FIG. 4A to 4D.
Typically, the metal c-ring seal 6a is less than 1-5 mm in thickness cross section. A typical diameter of the metal c-ring seal for use on a stinger 1 can have a diameter of 38 mm and a cross section “diameter” of about 1.6 mm, without such dimensions being any limitation, only mentioned as an example for the proposed stinger 1. The metal c-ring seal 6a is typically made of a corrosion resistant alloy, such as Inconel. Inconel 625 and Inconel 718 are examples of suitable grades. The metal c-ring seal 6a may have very thin walls and may typically have a wall thickness t2 in the range of 0.01 to 0.25 mm.
With reference again to FIG. 4A to 4D, FIG. 4A shows the metal c-ring seal 6a supported in the shoulder seat s in the transversally extending support surface 1b′, which provides a safe and controllable support for the metal c-ring seal 6a. Arrows R indicate that the U-seat with short unsupported length is stable over pressure cycling shown by arrows P.
FIG. 4B shows the metal c-ring seal 6a supported only along a line in the transversally extending support surface 1b′, which provides an unstable and poorly controllable support for the metal c-ring seal 6a. The arrows R indicate that the ring thickness now is insufficient to withstand pressure, shown by arrows P, over the unsupported length.
FIG. 4C shows the metal c-ring seal 6a supported in the transversally extending support surface 1b′. The arrows R indicate that the pressure, shown by arrows P, plastically deforms the metal c-ring seal 6a. 
FIG. 4D shows the metal c-ring seal 6a supported in the transversally extending support surface 1b′. The arrows indicate loss of sealing after pressure removal.
Thus we may say that the metal c-ring seal 6a operates like a normal lip seal and that is how it is used in most applications. We repeat that specifically for this application it works because of:                (a) small size to achieve the required stinger envelope        (b) thin section material to make the seal flexible allowing manufacturing tolerances that can be reliably machined        (c) u shaped seat that supports the c-ring against pressure.        
As mentioned, with a normal flat seat behind the metal C-ring, the pressure tries to make the c-ring go a square shape. The effect of this is that after pressure application, the metal c-ring seal is permanently deformed and loses contact preload between the surfaces being sealed unless the material thickness is sufficient to avoid plastic deformation and damage.
The U or V shaped seat will support the metal c-ring seal and provides for that the seal cannot be forced by pressure into a square shape. Therefore a thinner than usual metal c-ring seal 6a can be used and thus obtaining the benefit (b) given above.
As mentioned, the metal c-ring seal 6a is very slightly deformed if installed at tolerance extremes. Some further small deformation does occur with initial pressure application, but after this the metal c-ring seal 6a operates in an elastic regime. The metal c-ring seal 6a can be re-used in theory since these deformations are extremely small when compared to the seal interference with its sealing surfaces.
Thus it is to be understood that without support from the U or V shaped seat s, the thin section c-ring is deformed by pressure and loses ability to seal after one cycle.
This would require a thicker ring which cannot be made so small and is more sensitive to tolerances, making a less robust seal that is more expensive due to much reduced tolerances on mating parts.